I have been reading quit a bit on the topic but I'm still somewhat confused... In my case I have an experiment with animals that was replicated 3 years. Ten animals are used in each year for each of two treatments; at the end of the year they are replaced with new but similar animals. The variable of interest is the animal's weight. Weights were recorded for animals in two Treatments, at irregularly spaced (Time) intervals (intervals vary between years, but not within years) . My main interest is the weights evolution over time, i.e. the regression of weight on time and treatment, but I also should lke to determine if Year had an effect. My model then should be (?) Weight= Treatment Time Treatment*Time; I suspect that the random statement should be something like: random intercept AnimalNumber Time / subject Year; I have read that the covariance structure in the random statement above should be specified by type=sp(pow) given the irregular time intervals covtest 'random slope coeff=0' ...0; I would certainly appreciate feedback on the above. Thanks
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